The sex lives of animals: A rough guide

The urge to reproduce, to pass on our genes to a new generation, is one of the most powerful in our lives. This is equally true of the bewildering variety of animals with whom we share the planet. And they have an equally bewildering variety of ways of going about it.

So, here is a run-through of the huge diversity of sexual behaviours found in the animal kingdom. We begin with behaviours that are now fairly widely accepted in human society, such as promiscuity and homosexuality, before turning to stranger acts, such as necrophilia and bondage.

Promiscuity

The theory of evolution predicts that, in many cases, promiscuity can be a good survival strategy. By mating with as many females as possible, males maximise the number of offspring they produce, and thus their chances of passing on their genes.

By contrast, because of the high cost of reproduction, females may be extremely choosy, and only mate with the "best" male available, or with one who is willing to make a large investment, such as building her a nest. Still, some females are promiscuous, and the exhibition The Sex Lives of Animals sought to overturn these sexual stereotypes.

For many species, particularly if the breeding season is short, promiscuity is the best way to pass on your genes.

Red-sided garter snakes famously engage in what can only be described as orgies, forming large "mating balls" of writhing snakes.

Toads mate in a position called amplexus, in which the male mounts the female from behind and fertilises her eggs externally.

Promiscuous sex can also be a form of social bonding. This is most famously observed in bonobos, a kind of chimpanzee. Members of a bonobo troupe have sex with each other in pretty much every combination, and engage in a host of different sexual activities. Their societies are female-dominated and seem to be almost entirely non-violent.

Masturbation

There is a joke that 99% of humans masturbate, and the remaining 1% are liars. Of course, many animals are prone to indulge too.

Monkeys are notorious for it. The story that taking granny to the zoo because it'll be a safe and anodyne day out, only for her to be horrified when the monkeys start masturbating, is firmly grounded in reality. There are so many videos along these lines on YouTube, one hardly knows which one to pick.

There is some evidence for concrete benefits. For example, spontaneous erections and masturbatory behaviour are common in stallions, which horse breeders try to prevent. However, these attempts not only fail to work, but actually have an adverse effect on the horses' semen.

Among the more recent examples are the fabulous New York Central Park zoo penguins, Roy and Silo, two males who, after trying to incubate a rock, were given an egg and together raised the resultant chick, named Tango.

Not to mention the 1994 observation of two deep-sea octopuses mating. Researchers who studied the video realised that both individuals were male, and that they were actually from different species.

Transsexualism

Changing sex is a tricky business for humans, involving hormone injections and surgery, but some animals make it look easy. Transsexualism is particularly common among simple creatures like worms and slugs, but also among fish.

Some species engage in "sequential hermaphroditism", in which they start out one sex, but switch later in life. One example is a marine worm called Ophryotrocha puerilis puerilis, which starts out male but may become female. Large males are particularly likely to do so, as being large is an advantage for the females, but not the males.

Sex changes don't have to be one-way. When two female gobi fish meet, one of them will often become male so that they can mate. Switching back is less common, perhaps because there are higher costs involved, but it does happen.

Many animals have their cake and eat it by practising "simultaneous hermaphroditism" - they have both male and female sexual organs. Snails and earthworms both do this. Sea slugs have it down to a fine art, engaging in "sperm trading" to ensure that both partners get their fare share.

Self-fertilisation is less common than mutual mating - neither snails nor earthworms do it. Banana slugs, which live in North America and reach lengths of 25 centimetres, have no such inhibitions and regularly fertilise themselves. Only one fish species, the mangrove killifish, is known to self-fertilise.

Part 2

We now move on to sexual acts that many of us would consider "alternative", even perverse.

It is difficult to equate the behaviours observed in animals with those humans engage in. Humans who indulge in these activities often do it partly for the thrill of breaking taboos, or because they delight in doing things they believe to be morally wrong.

By contrast, animals doing them may simply be making mistakes, or attempting to survive in difficult circumstances. It is unlikely that they have the same complex emotional response to, say, bondage, as a human has.

Violence and forced sex

It should come as no surprise that sex in the animal kingdom is often rough and dangerous. Female praying mantises are the poster child of dangerous sex, thanks to their predilection for biting off their mates' heads immediately after copulation (watch a video) - but many seemingly mild-mannered creatures can also be pretty violent in their sex lives.

For instance, short-tailed shrews are not normally thought of as a brutal species. However, if a male persists in attempting to mate with an unreceptive female, a fight may break out - and such fights can end with one of the shrews being killed and eaten by the other (pdf format, see page 11).

Weapons are not out of bounds in the animal world, either. Male guppies mate using a hooked appendage called a gonopodium. This maims the female, causing inflammation which locks the male's sperm inside. Male guppies will also mate with females of another species called Skiffia, which have no defences against the damage the gonopodium can cause.

Some sort of prize ought to be given, however, to the flatworm Pseudoceros bifurcus. These hermaphroditic worms simply stab one another with their penises, leaving both parties severely punctured.

Bondage

Among humans, tying up a sexual partner is a form of thrill-seeking, but for male spiders it can be a life-saver.

Mating among spiders is notoriously tricky, as the male is often smaller than the female and may well end up getting eaten. Males have developed all manner of techniques to get around this problem, including playing dead.

But one of the sneakiest tricks is deployed by, among others, European crab spiders. The males tie down their prospective mates with silk threads to ensure they don't become a post-copulation snack.

An unfortunate male duck had crashed into the window of the Rotterdam museum - at which Moeliker worked - and lay dead on the ground. At this point another male approached and mated with the dead duck for over an hour.

Later investigation suggested that the duck was on a "rape flight". Male mallard ducks will often force females to copulate with them, and when several of them converge on the same female she may not survive the experience. In this case, the male may have applied the same instinct to the dead mallard (read Moeliker's paper).

A 1989 review of the human psychiatric literature concluded that the most common cause of necrophilia is not a specific attraction to corpses, but simply "possession of an unresisting and unrejecting partner". Given this, it would seem the duck's behaviour may have been all too human.

Bestiality

From a practical point of view, we can define bestiality as the practice of one animal mating an individual from different species, with which it is not genetically compatible. Some instances can, of course, be put down to mistakes, particularly when species appear similar, but there are plenty of examples where the animal has, quite simply, no such excuse.

As with necrophilia, such behaviour seems to be an evolutionary mistake. By expending sperm without any possibility of producing offspring, the animals are reducing their fitness.

The unfortunate male wasps duped into ejaculating on orchids are one of the most striking examples of this. The orchid benefits by spreading its pollen, but it seems the wasps get nothing out of it.

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